1. not planned or arranged in advance; happening without being on a schedule

Examples

• The flight was delayed due to an unscheduled maintenance check.
• We made an unscheduled stop at the roadside diner.
• The president held an unscheduled meeting with reporters.
• An unscheduled power outage affected the entire neighborhood.
• The teacher allowed unscheduled breaks during the long exam.
• The train made several unscheduled stops along the route.